On my 2002 CSFE I have a "kneeler". It drops the right front air airbag's air so the steps are closer to the ground to help people get on easier. When it is activated the throttle is dissabled as a safety feature so you cannot drive off with it down. Not likely you have this but you never know. There is a large toggle switch on the left panel near your thigh clearly marked Kneeler and up and down.

No kneeler on this one.
I did find one small air leak but haven't found the reason why the compressor isn't building enough pressure. I also pumped the air tank to 90 psi but nothing happened. Rear air bag aren't getting inflated. Need to check how the logic for filling those up is.

air suspensions are notorious for leaking.. Ive replaced the air springs and air valve on one of my busses.. it wasnt hard and its not real expensive to do so..

there is a lever valve connected to the air bags and mounted along the rear frame member.. a lever connected to the moveable part of the suspension opens and closes the air valve..

coming from your auxiliary air tank (your bus may have 3 tanks or it might have a single tankl with 3 sections.. a protection valve will come off of the tank.. then an air hose to the rear suspension valve.

first the simpole stuff, be sure all the drain valves on the bottom of the tanks are closed and not spewing air out.. its easy to knock one of those loose and have it leak air out.

secondly if its only getting uo to 30-40 PSI you should be able to hear air. while the bus is running.. if you crawl around to the 4 corners and look under the hood and hear no air.. (and the compressor pulley is in fact spinning and not slipping at the belt).. then the governor may be suspect.

it is a little cylindrical shaped device usually mounted to the compressor with one or 2 small air lines going to it.. you can tap on that governor with a mallet or plastic hammer and it may free-up.. if thats the case and you then build pressure, change the governor its shot.

if you hear hissing from around the air suspension (engine running). then you can disconnect it for testing. unscrew the aur line from the protection valve and cap it off. most air suspension air line ports are 1/8" NPT and you can buy a 1/8 NPT plug at home depot and screw in the port.. if you build full pressure then you know your issues is in the air suspension and can troubleshoot back there..

you can drive a bus short ways with a flat air suspension.. it will ride terribly and I wouldnt do it long. but it can be done. I did with mine till I got it home and replaced my air springs.
-Christopher

if the air hose is filling the aux tankl that is correct it wont fill the others.. but if the air hose is filling the wet tank that should be filling the other 2 tanks.. there are check valves so that the 2 tanks past the wet tank (first tank) do not backfill the other..

I am going to first look at the governor.
There is no air hissing and the system only loses about 1 psi per hour.
The compressor on this bus is gear driven. Not belt. In addition, I believe the power steering pump runs either on the same compressor shaft which extended behind it, or is somehow coupled to it. At any rate its not easy peasy to get the whole shabam out of there.

So this happened to me on my e450 short bus about a month ago after a big rain. My bus started but no throttle response. I could actually get a response if I stomped the pedal then backed off a little I could keep it going but a stomp was nessecary. Sooo I donít have air brakes ....... I drove to a mechanic and we unplugged the sensor at gas pedal and two other things and when we plugged them back in it went away Iím sorry I never figured out he problem but if this helps mine was completely electric I think the mechanic said there were 3 sensors with 5 volt maybe ?? Not sure but when he plugged in the scanner it gave him 3 options to work with and kind of had to eliminate one by one. Mine was throwing a service engine soon light.

This problem has been fixed.
Fore reference, one of the pressure protection valves was leaking and a rear air bag.
In addition, the throttle control on this bus does have a safety mechanism preventing throttle actuation below a certain air pressure. You can circumvent the mechanism by continuously pressing the " brake bypass" button on the control panel this will let you actuate the throttle pedal and it will fill up the air tanks quicker than just idling.